With a Type tool and exactly one character selected, the canvas shows up
to five of that character's alternate glyphs in a row beside it. A click
runs the command the Glyphs panel's double-click runs, as one undo step;
an arrow at the end opens the Glyphs panel on Alternates for Current
Selection when the character has more. Preferences > Type > Show
Character Alternates (showCharacterAlternates, on by default) turns the
row off.